RollbarHandler should include extra data from LogRecords automatically (or provide a switch to enable that)

[GrahamLea]

Expected Behaviour

I've integrated Rollbar with Python's standard logging module using the RollbarHandler.
Some of my logging statements use the extra keyword parameter to add extra context, e.g.

    except:
        logger.exception("Uncaught error", extra={"customer.id": customer_id})

I expected RollbarHandler would include the data in the extra argument automatically, but it doesn't.

Actual

RollbarHandler looks for an attribute called extra_data on the LogRecord, which isn't a standard part of the logging API that I can see.
While pyrollbar provides a way to transform the payload, the LogRecord isn't available to that callback, so there's no way to get hold of the extra data at that point.

Workaround

I was able to put the extra_data attribute that RollbarHandler is expecting into the LogRecord by providing my own subclass of Logger and overriding the makeRecord function to add it in:

        if extra:
            record.extra_data = extra

Seeing as the purpose of RollbarHandler is to integrate with the logging module, it feels like this step shouldn't be necessary on my part. I think RollbarHandler should be figuring out from the LogRecord's __dict__ what data was added via the extra parameter and including this in the custom data.

Environment

Python 3.9.6 (v3.9.6:db3ff76da1, Jun 28 2021, 11:49:53) [Clang 6.0 (clang-600.0.57)]
pyrollbar===0.16.1

[GrahamLea]

Related to #381